Conventionally, in the field of copiers and printers, various light beam scanning devices have been provided as means to write an image on an electrophotographic photosensitive member. Among these, an optical device of a multiple beam type has been developed to write an image of a plurality of lines in a single scan by simultaneously emitting a plurality of beams and deflecting each beam so as to scan adjacently with predetermined spacing in a subscan direction to improve image density and improve image writing speed.
This type of optical device is constructed to form an image using a plurality of light beams such as disclosed in Japanese Laid-Open Patent Application No. 6-250105 which describes art to equalize beam spacing in a subscan direction by arrangement of the light source.
In conventional multi-beam type optical devices, however, a disadvantage arises insofar as jitters occur at the image edges (i.e., edges on the upstream side and downstream side of the scan) even with accurate beam spacing in the subscan direction.
That is, when light beams B1 and B2 irradiate the surface of a photosensitive drum 27 at predetermined spacing in the subscan direction, as shown in FIGS. 7(A) and (B), the B2 which is supposed to irradiate a point A1 actually irradiates a point A2 at the image edges based on the curvature of the photosensitive drum 27 such that the main scan magnification changes and results in jitter G.